linux-kernel - Re: [PATCH] sched/fair: provide u64 read for 32-bits arch helper

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Message-ID: <20200728111302.GV119549@hirez.programming.kicks-ass.net>
Date:   Tue, 28 Jul 2020 13:13:02 +0200
From:   peterz@...radead.org
To:     Vincent Donnefort <vincent.donnefort@....com>
Cc:     mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        lukasz.luba@....com, valentin.schneider@....com
Subject: Re: [PATCH] sched/fair: provide u64 read for 32-bits arch helper

On Mon, Jul 27, 2020 at 04:23:03PM +0100, Vincent Donnefort wrote:

> For 32-bit architectures, both min_vruntime and last_update_time are using
> similar access. This patch is simply an attempt to unify their usage by
> introducing two macros to rely on when accessing those. At the same time, it
> brings a comment regarding the barriers usage, as per the kernel policy. So
> overall this is just a clean-up without any functional changes.

Ah, I though there was perhaps the idea to make use of armv7-lpae
instructions.

Aside of that, I think we need to spend a little time bike-shedding the
API/naming here:

> +# define u64_32read(val, copy) (val)
> +# define u64_32read_set_copy(val, copy) do { } while (0)

How about something like:

#ifdef CONFIG_64BIT

#define DEFINE_U64_U32(name)	u64 name
#define u64_u32_load(name)	name
#define u64_u32_store(name, val)name = val

#else

#define DEFINE_U64_U32(name)			\
	struct {				\
		u64 name;			\
		u64 name##_copy;		\
	}

#define u64_u32_load(name)			\
	({					\
		u64 val, copy;			\
		do {				\
			val = name;		\
			smb_rmb();		\
			copy = name##_copy;	\
		} while (val != copy);		\
		val;
	})

#define u64_u32_store(name, val)		\
	do {					\
		typeof(val) __val = (val);	\
		name = __val;			\
		smp_wmb();			\
		name##_copy = __val;		\
	} while (0)

#endif